Relationship between trunk and pelvis coordination strategies and stances during tennis forehand strokes

The forehand is the most common groundstroke, yet stance-related pelvis-trunk mechanics during acceleration remain unclear. We compared open (OS), semi-open (SES), and square stance (SS) forehands in 25 male competitive tennis players (21.3 ± 2.0 years) performing maximal-effort flat strokes, with three-dimensional kinematics recorded at 200 Hz. Signed peak X-factor, forward momentum, angular-momentum peaks, peak delay, and continuous relative phase (CRP) variability were analysed using repeated-measures ANOVA. Time series of pelvis and trunk angular momentum and CRP were analysed using one-dimensional statistical parametric mapping (SPM1D). Stance affected peak delay, signed peak X-factor, forward momentum, and peak pelvis angular momentum, but not peak trunk angular momentum. Mean signed peak X-factor was negative in OS and SS but positive in SES. OS predominantly showed pelvis-first angular-momentum peaks, whereas SES and SS predominantly showed trunk-first peaks. SS showed the greatest forward momentum. During early acceleration, pelvis and trunk angular momentum were greater in OS and SES than in SS. CRP differed significantly among stances during 14.3%-62.5% of acceleration, with post hoc comparisons indicating greater pelvis phase lead in OS and SES than in SS. These patterns may inform stance-specific biomechanical assessment. Their associations with ball speed and stroke performance remain untested.